The present invention relates to MPX signals, and more particularly to a carrier generation facility for a demodulator thereof.
MPX signals are used in the transmission of frequency-modulated stereo broadcast signal in the VHF range or in the transmission of television sound according to the BTSC standard for example. Besides the sound information, the MPX signal may contain additional information, such as the Automotive Radio Information (xe2x80x9cARIxe2x80x9d) and/or Radio Data System (xe2x80x9cRDSxe2x80x9d) information. The frequency schemes of the various MPX standards differ only slightly. Generally they contain, at baseband, a sum signal R+L, a pilot signal above the sum signal, and a suppressed-carrier amplitude-modulated difference signal Lxe2x88x92R at twice the pilot frequency. Above the difference signal, there may be additional signals, particularly around three times the pilot frequency. The contents of the individual types of additional information are inserted in the MPX signal by single-sideband modulation or, particularly in the case of digital information, by quadrature modulation, for example.
A distinction between the individual MPX standards is possible via the frequency of the respective pilot signal, which is 15.734 kHz in the case of television signals according to the BTSC television standard and 19 kHz in the case of FM radio. For the demodulation of the individual signal components, which is done by mixing, carrier signals of correct frequency and phase must be provided in the demodulator, with the respective pilot signal serving as a reference signal. For digital demodulation, digital carrier signals are used whose values at the sampling instants correspond to the values of analog carrier signals. An example of a digital modulator can be found in a data sheet of ITT Semiconductors entitled xe2x80x9cCAP 3540B, CAP 3541Bxe2x80x94Car Audio Processor Hardwarexe2x80x9d, Edition May 22, 1997, Order No. 6251-434-1PD. The processing clock frequency in the monolithic integrated circuit of that demodulator is locked to the frequency of the pilot signal.
It is an object of the invention to provide a digital demodulator which processes different MPX standards with minimal additional circuitry, with the clock rate of the digital carrier signal, and thus the clock rate in the digital demodulator, being independent of the frequency of the pilot signal such that arbitrary clock systems may be used.
Carrier generation facility for a switchable digital demodulator (D) of digital MPX signals (mpx1) with associated pilot signal (pi) which are locked to an arbitrary clock signal (t1). A pilot signal PLL (10) generates a first carrier signal (x1.1, x2.1) and a second carrier signal (x1.2) by means of a first value allocator (15) and a second value allocator (20), respectively. According to the MPX signal (mpx1) to be demodulated, a control device (60) delivers a start value (i0) and at least one phase correction value (k1, k2). The start value (i0) sets the capture range of the pilot signal PLL (10) for the pilot signal (p1) according to the respective standard. In a first correcting device (16) and/or a second correcting device (23), the phase correction values (k1, k2) correct the system-inherent phase deviations of the first carrier signal (x1.1, x2.1) and/or the second carrier signal (x1.2).